What is sound?

What is sound? Next time you're out  for a walk close your eyes and listen. What can you hear? Maybe it's cars driving past you, maybe it's a bird flying through the air, or maybe it's you whistling as you walk. These are all examples of sound, but what is sound? Sound is a kind of wave. No not like that. It's more like a ripple that moves from one place to another. Okay sort of like that. Think of a stretched slinky. If you give one end of it a flick, you can make a pulse move along to the other end. Sounds a bit like this. Actually sound isn't quite like this. Instead of flicking the end of the slinky up and down, sound waves are more like giving the slinky a push and a pull. When things make sounds, tiny bits of stuff in the air called particles get squashed together, or compressed, and get stretched apart, or rarefied. This pattern of squashing and stretching  carries on through the air between particles, as they bump into each other until we hear it with our ears. This is what we call a longitudinal wave. But it's usually easier to think of sound waves wiggling up and down instead. This is what we call a transverse wave. Now  let's pause our sound wave for a moment. We can change the shape of the wave to affect  how it sounds. By changing the height of the wiggles we can change the volume. If we make the wiggles bigger, the sound gets LOUDER. If we make the wiggles smaller, the sound gets quieter. We call the height of the wave its amplitude. Or we can change how long or short  the wiggles are. If the wiggles get closer together, the sound gets higher. [Rising tone] If the wiggles get further apart, the sound gets lower. [Falling tone] This is called the frequency of the wave. But how do we make sounds? There are four main ways of making sounds. The first one is that we can knock two hard objects together, like claves. [Knock from 'Frozen'] "No!" The second one is that we can pluck or hit a  stretched string, like in a piano, guitar, or ukulele. ['Duelling Banjos' plays on ukulele] The third one is that we can hit a  stretched sheet or membrane, like a drum. [Drumroll] And the fourth one is that we can blow  air through a pipe, like in a saxophone. ['Baby Shark' plays on saxophone] "No!" See if you can think of different musical instruments and work out which way they make sounds. To learn more about sound and hearing, check out the Ear Facts page on the University of Nottingham website. There you'll find lots of information along with some fun activities. You can also submit a question about sound and hearing  and get answered by a real scientist. And you might even be able to find out how you can make your very own musical instrument out of a carrot. ['Shave and a Haircut' plays on carrot]